ABSTRACT The long term goal of this research proposal is to determine the mechanisms responsible for the important functions of TNF receptor associated factor 3 (TRAF3) and NF-?B inducing kinase (NIK) in regulating host defense against viral infections and autoimmune responses. We identified TRAF3 over 20 years ago as an adaptor molecule associated with a subset of TNF receptors including CD40, BAFFR and LT?R. We subsequently generated TRAF3 knockout mice and found that they die within two weeks after birth with over- reactive inflammatory responses. In the past several years, we have defined a TRAF/cIAP/NIK complex responsible for suppressing non-canonical NF-?B activity in unstimulated cells through constitutively degrading NIK. The significance of our findings is supported by the observation that mutations in the genes encoding components of the TRAF/cIAP/NIK complex are associated with inflammatory and autoimmune diseases, as well as several hematological cancers such as multiple myeloma and diffuse B-cell lymphoma. Our recent studies have also discovered a novel NIK/IKK/CRL protein complex, which acts as a negative feedback control in preventing over-reactive non-canonical NF-?B activity after receptor activation. In addition, our preliminary studies have identified a novel NIK/STING/TBK1 complex, which can enhance DNA-induced Type I interferon (IFN-I) production. Based on our genetic and biochemical studies, we found TRAF3 has opposing roles in regulating DNA vs. RNA-induced IFN-I induction. We have provided evidence that crosstalk between the non- canonical NF-?B and IFN-I indcution pathways may play important roles not only in regulating host defense against DNA virus infection but also in DNA and BAFF-mediated syngergistic pruduction of IFN-I and auto- antibodies, which are a hallmark of certain autoimmune diseases such as Lupus. Our overall hypothesis is that TRAF3 and NIK play important roles in regulating anti-viral immunity and auto-immune responses through controlling non-canonical NF-?B activation and its crosstalk with IFN-I production. Our goal is to gain a functional and mechanistic understanding on TRAF3 and NIK in regulating non-canonical NF-?B activation and IFN-I production. In Aim 1, we will define the major components of the NIK/IKK/CRL complex and determine their roles in the negative feedback control of the non-canonical NF-?B pathway. In Aim 2, we will define the major components of the NIK/STING/TBK1 complex and determine their roles in regulating the crosstalk between non-canonical NF-?B and IFN-I induction pathways. In Aim 3, we will determine the contributions of the crosstalk between non-canonical NF-?B activation and IFN-I responses in host defense against DNA virus infections and its association with autoimmune diseases. Finally, we will explore the possibility of using small molecular regulators of the non-canonical NF-?B pathway as novel agents to protect viral infections and treat autoimmune diseases. We believe our proposed studies will assist in future attempts to pharmacologically intervene against infectious and autoimmune diseases.